Tone keyer control circuit



J 311, mm w. H. BLISS 2,

TONE KEYER CONTROL CIRCUIT Filed Dec. 14, 1945 5 Sheets-Sheet 1 KEYED 761v! OurPur INVENTOR M4225 Buss ATTO R N EY Ja. 31, 1950 w. H. BLISS TONE KEYER CONTROL CIRCUIT Fil ed Dec. 14, 1945 TRAMsFoeMm 29 He. 1

3 Sheets-Sheet 2 To A'A/apE 0/ 720005 42 F/etZ INVENTOR WARREN H Buss ATTORNEY w. H. BLISS TONE KEYER CONTROL cmcurr Jan. 31, 1950 s Shets-Sheet 3 Filed Dec. 14, 1945 INVENTOR h/mes/v hf Buss ATTORNEY Patented Jan. 31, 1950 TONEKEYER CONTROL CIRCUIT Warren H". Bliss;v Riverhead, N. Y.,. assignor to Radio Corporation: of. America, a corporation of Delaware Application Decemberl i, 1945, Serial No. 635 086 11'. Claimsn (Cl. 178-66) This application relates; to tone; keyers? for? tealegraphy systems and: discloses: an. improved? means for" converting: the output: signal: from a. radio telegraph receiver: into: keyed; tonev form for transmission to a: radiot-termi'nalf. for recordingzorfor other utilization:

The tone. keyer of my-presentinvention is; for; use with receivers of telegraphy" signals: of: the; off-on typeor of the frequencyshifttypevorrsimilar signals.v In. systems: as: involved herethe receiver has an output-in. the z form-.of. voltagewhich rises and falls with respect to a base: value in accordance with. the keying; In manyembodiments the output voltageris derivedfroma diode rectifier andvariesin anegative direction inthe presence of marking condition in: the received wave and back toward zero. when the" mark. con dition is changed tospace'. Moreover; in. these known systems this varying; voltage with; a. direct current component is. used. to. operate: a trigger;- ing or'locking circuit which in. turn: causes. definite on and off keying of aitone' stage; thereby supplying keyed. tone. to? the radio terminal to operate a printer or for'othersuse;v

The diode rectifier pulsating: voltage: output usually is accompanied 'by: components represent; ing noise on the receivedtelegraphysignalorset' up in thesyst'em', and iinamany." OfitheS'e known receivers" and. recording means: the noise" com"- ponents at the-rectifierrcutput in? the absence. of marking currentsin' there'ceiverare sufiicientzto operate thelockingior; triggerin circuit thereby. sending out keyedf tone: Moreover; the noise components: on;v the rectified 1 output in the:- pres.- ence of signals aresoiten'sufficientttooperate; the triggering or locking: circuit: to? thereby-turn off the' tone prematurely: The: general object" of the presentiinvention .isprov-isionzof. a tone keyer free of the defects mentioned above. This object is accomplished by providing: means in the triggering or looking: circuit. andrithe' control apparatus thereforv for: holding: back" or preventin the. noisecomponents riding": onthe marking voltage from turningroirortripping th-elocking circuits from the: positionswfrom.whichthey, were tripped. by reception of? the. marking voltage.

A: more' specific object of my invention is: an improved triggering orlocking'rcircuit .and' a; conetrol apparatus therefor; wherein: means is provided' foradjustingitheiexactpointzattwhichatrig gerin action takesplaiceeonztlie appearance of marking" voltage in'1the reoeive1"output: This permits location of "the t'hresh'oldipoint';at?v which markingcvol-tage' takes: efiectztobeia-djusted above the output noiserleve-l:inztheipresencezof marking: In. my. improved. system means: is: alsa provided for'holdingback; the tripping-1' action when the receiver output markin'gz'voltagei falls off 1' due; to I noise component thereon until. a; lowcrrvalua-Qf; inputzsignaliisaattainedi. Thenzthatonesisszturned '1 received signal.

off at a signal value much lower than the-signal. valueat which thetone was: turned on. Asa:

result of. this improved hold-back. functionmorc:

noise and interference. may bgtolerated onthe fie'd detector circuit to be used at the receiveroutput in Figs. 1. and"; 2.- to supplythe control potentials; while Figs. 3a; 3b, 3c, 3d, 3e-and 3f are oscillation or pulse voltage diagrams used toillustrate the-operation of mysystem- Referring to Fig- 1; elements I and. 2 indicate a conventional antennaand radio receiver'systern having a detector with diode output atterminal 3 Which is negative.-. If frequency shiftsig-nal line; is being carried out. the diode arrangement and connections may'beas indicated in Fig..1a2 For on-off carrier frequency telegraphy the. diode connections may be as shown in Fig; 1. The

vacuum tube triode. 9 hasits control grid. cone nected over resistor Std-terminal 3 by a variable resistance 6 and to the anode of-triode I 5 through resistor II. The resistor 5 may be the receiver diode load resistance or connected therewith. A resistor I0 and a potentiometer 1 associated with the positive terminal of adirectcurrent plate suppy source and a battery 8 provides'a voltage divider and voltage adjusting-means for varying the potential at'the' cathode of the triode 9.

The triodes !5 and I8 are connected in an Eccles-Jordan locking circuit .with anode resistors l2 and 22 connecting the anodes to thadirect current source andwith resistorsl3 and I4 crosscoupling the'control grids of the:tubes. The control' grid of triode Iiis. also connected to the anodeof' tube 9 whiletube I8 has a grid resistor I1; and the cathodes of both tubes I5 and I s are connected'toground by-a.comrnon cathoderesistor 16. The tubes I5 and I8 as connected provide an Eccles-Jordanlocking circuit broadly similar to that disclosed in Finch U. S. Patent #1344950; The-arrangementis such that when current is started or caused to'flow in. tube IS the rid of tube l8 becomes less positive byvirtue of its connection through resistance I4 to the anode" of tube l5-and current iscutoiT in tube l8; When current-liscaused todecrease intube I5: by converse action current is=cut off'in tube l5'-and;tube I8 becomes fullyconductive.

Resistors I 9 and-2!] .comprisea potentialdivider' across: the. anode and cathode. impedance of l the lockingrtubevstagelfl andraraused .tocfeed poten- Viewed from another aspect. a;-

when the signal-to-noise ratio is poor.

3 senting its output to the cathode of the diode tube 2|. Resistors 23 and 24 form a second potential divider across the main direct current source for supplying a positive voltage of suitable value at the point 58, and through series resistor 25 to the anode of diode 2|.

The tubes 3| and 32 have their control grids coupled differentially by a transformer 29 to contacts 28 to which alternating current of tone frequency may be applied. The anodes of the tone amplifier tubes 3| and 32 are coupled to the primary Winding of a transformer 33, the secondary winding of which supplies the keyed tone output at terminals 34. Fixed bias is provided for the control grids of tubes 3| and 32 by the potential divider comprising resistors 21 and 30. Note that the cathodes are positive with respect to ground. Lead 26 from the anode of the diode 2| supplies a signal voltage from the anode of the diode 2| in like phase to the control grids of the tubes 3| and 32 to make the same conductive or non-conductive, depending on whether or not the locking circuit output represents mark or space.

In operation the output of the receiver diode,

that is, the input at points 3 and 4 may comprise alternating voltages of the form shown in Fig. 3a

It is desired to obtain the keyed tone wave shown by Fig. 3f. Portions of the line 45 of Fig. 3a desig- 'nated at 46 indicate the no-signal on, or the noise condition, while the portions at 41 designate the signal-on or signal-and-noise condition. The output at terminal 3 of receiver 2 is derived from the anode of the diode and is negative. This negative potentialis supplied to resistor 5 which may be the receiver diode load and by resistor 6 to the grid of the tube 9 so that this tube 9 is only moderately biased in the negative direction by the noise 46 during the no-signal condition. This moderately negative bias allows triode 9 to be in a state of conduction so that the potential on the grid of tube I5 connected to the anode of tube 9 is held at a relatively low negative potential. This cuts off the tube I5 so that its anode potential is relatively high and this relatively high anode potential is supplied to the grid of tube I8. Hence, tube I9 of the locking circuit is on or conducting when tube I5 of the locking circuit is off or non-conducting.

When the signal comes on, the output of the diode of the receiver will swing more negative and will be as represented at 41 in Fig. 3a. This condition will exist at the terminals 3 and 4 and across resistance 5 to swing the grid of tube 9 negative and tube 9 will be cut off due to this greater negative potential applied to its grid. The threshold point to which the signal and noise represented at 41 must rise in the negative direction to cut off the tube 9 may be at the level 48 of Fig. 3a.

When tube 9 ceases conduction the control grid of tube I5 is caused to go more positive, since it is connected to the anode of tube 9 and the states of conduction of the tubes I5 and I8 will be reversed, leaving tube I5 conductive and tube I8 non-conductive.

" The cutting off of current through tube I8 will permit its anode potential to rise in a positive direction thereby causing the cathode of diode 2I to rise in potential. This biases diode 2| beyond cutoff and allows the potential on lead 26 to rise to the value which is maintained at point 58. Since this rising potential is applied to the control grids of tubes 3| and 32'the same become operative to amplify tone applied at 28 and supplied to the outputs at 34. The amplified tone,

' which may be as represented in line 3f, is turned explained in detail.

on and off according to Fig. 3b, which represents the potential variations on lead 26 as governed by mark and space conditions.

-my improved circuit holds back from triggering until a lower threshold level 49 is reached. To do this I provide improved means for preventing the triggering circuit from tripping back to the no-signal condition at which I8 is conductive and I5 non-conductive until a selected threshold level such as at 49, Fig. 3, is reached. This is accomplished by the action or function of resistors 6 and II. Under the no-signal condition, that is, at 46, Fig. 3a, a certain amount of positive bias is applied to the grid of tube 9 by resistor II from the anode of tube I5 which under no-signal condition is out off. When the signal arrives and the grid of 9 is biased negative to cut tube 9 off and start conduction in tube I5, the potential at the anode of tube I5 drops and this drop in potential is applied to the grid of tube 9 to drive the same in a negative direction even further below cutoir than the marking signal causes it to go. This additional bias or greater than cutofi bias persists during the signal-on condition (at 41 in Fig. 3a) and, hence, sets a lower threshold value as illustrated by level 49 of Fig. 3a to which the signal level must fall before tripping action takes place, that is, before tube 9 is permitted to again conduct to cut off tube I5 and turn on tube I8. The signal plus noise drops below level 49 to trip the locking circuit of triodes I5 and I8 back to the off or no-signal status. This point at which thresholding action may take place is illustrated at point 5| of the varying voltage shown in Fig. 3a.

The potentiometer I provides a means for adjusting the potential at which the cathode of r tube 9 operates and thereby for setting the threshold level 48, Fig. 3a, at which the trigger circuit is in the signal-on condition with tube I8 non-conductive. The variable resistor 6 controls the hold-back effect or magnitude of change or dropping off in signal voltage from level 48 towards level 49 which must occur before the tubes I5 and I8 are triggered to the signal-off condition with tube I5 non-conductive and tube I8 conductive.

The diode rectifier 2| operates to clip or limit off any irregularities that may come from the anode of tube I8 during the signal-on condition when a stable tone output at 34 from the tone keyer is desired. During the signal-off time triode I8 is conducting and its anode potential is down to a relatively low value. This allows the cathode of tube 2| to assume a relatively low positive potential such that there is a flow of current from the point 58 down through resistor 25 through the impedance of diode 2| and resistor 20 to ground. This causes thepotential at the anode of diode 2| and on lead 26 and the grids of tubes 3| and 32 connected therewith to be at a value sufiiciently negative with respect to the potential on the cathodes of tubes 3| and 32 to cause these tubes to be completely cut off. This prevents the tone applied at terminals 28 from being passed on through amplifier tubes 3| and 32 to terminals 34. A rise in the anode potential of triode I8 ocicurring when-marksignal triggersthe the clrciiit on; raises the cathode potential 'oi diode 2- l above the cutofi value. "That is, it raises this cathode "to a potential "higher than=thepotential "existing *at the anode-Of diode -21 'and-atp0int58. The "potential -at point 58 is then supplied directly over lead 26 to the-gridsof 'triodes 3l and "32, gthere being no voltage 'dropinresistor 25. Tubes 31 and 32 are then turned a on to supplyton out- :put. 'Any "further variation in the potential of the anode of tube 18 except when-the'signal'goes iofi, ca-nnot'be passed onto the triodes'3-l -'and 32, 'and a constant level of keyed 'tone "outputis asfsured'during the mark. interval.

f In the "embodiment Ur -Fig.2, -a-fiip. fiop'typ .;of {triggering circuit is used in place of the Eccles- {Jordan locking circuit of Fig. 1. "Thearrange- "ment of Fig. 2 is in.many-respects'siniilartothe iarrangement of Fig. =1, and in Fig. 2 numerals corresponding to those used in Fig. "1 are "used in so far as possible, 'with the numerals being ;used' with primes in" Fig. 2.

The triggering circuit of the embodiment of Fig.2 comprises-tubes 36 and 42,'having anodes A and A" connected byresistors 3'1 and 38 to 'the positive "terminal of a source of direct 'current potential the negative terminal of-whichis grounded. The control grid of'tube '42 is connectedito ground'by resistors 49 and 4|;the re- .sistor 4'! being a 'cathode'resistor "common-to the cathode return circuits of tubes ,36 and .42. The tube .36 has its control grid grounded'by -a re- 1sistor'35 and its control grid directly coupled to the anode of the tube 9'. The .anode of tube'9' .is connected to the direct current source "by resistor' 59. The anode of 'tube' 36 coupled to'the control grid of the tube 42-by a condenseri39'the size of which controls in part at least the 'fres'sistor MEI, rthereb'y rcausing the potential -.-on :the cathode'of tube 36itoxrise. 'Tubei36 cannot fire 'or conduot agaimuntil condenser .39 is sufficiently chargedthrough resistor 31. This latter charg- ?510 eance f3 between'th e anode of tube 42 and grid of tube36vmay alsobe used. ThenwVhenthe tube :42 .is=turne'd ofi thepotential on the grid of the t'ube 36 surges uptoiturn this-tube on and the 'fiip-fiop action takes place as described above {until signal-cit condition makes tube 9 conydu'ctive.

The -flip-flop circuit -including tubes 36 :and 42 'continues to oscillate in the manner described above as long'as-thesignal persists above'the' level tratethis'performance, Fig. 3o'sh0wing theanode -fpot-entiai variation of tube '36 and Fig. 3d the 'varia'tionp'f anodepotentialof the tube '42. "It is preferable 'for "condenser '39 and resistor 40 to :25 have suchvalues'that during the signal-on condition when oscillations -arepcourring, tube 36 be in "the conduc'tive s'tate'a "much larger proportion of'the timethan tube 42. 'This'gives agreeiter average "value of =hold back voltage supplied $30 'ffromt-he-anode of triode 3l5to the-grid of triode 9. This "hold -back voltage is for the purpose "described'above-in connection with Fig. 1 to'hold oif 'the'triggering action until the signal magni- "tude 'falls-tosay-belowpoint'51 of Fig. Sato there- :535 "by prevent noise components riding on the sigquency at which the :tubes '35 :and 42 :oscillate or operate 'in the flip-flop circuit. The :anode or back voltage'supplied'fromtheanodeof tube 36 tube 42 'may be coupled to the grid :of tube 36 by a resistance l3f. Condensers 43 :and 44 are bypass condensers. The arrangement of "Fig. 2 .is otherwise substantially similar tothe arrangementof Fig. 1.

Theoperation of the embodiment of Fig.2 is .likewise quite like the operation of "the embodiment of Fig. 1 in many respects. The tubeQis again conducting in .the absence of signals as described hereinbefore, so that tube 3.6 is then non-conductive and tube 42 is then conductive. .Once the tubes 36 and '42 are triggered, "by Zthe :signal-on status, so that momentarily'tube "36"becomes conductive and tube '42 non-conductive, they oscillate continuously during th signal-on .condition,'in amanner similarto a multivibrator. The period at which they oscillate is determined .by charge and discharge rates of .oondenser"3.9 through resistors '31 and 40. When the signal mark is on andreaches a magnitude at-saypoint T59 of Fig. 3a, triode 9' is cut off. Itsanode potential rises as does the potential on the control grid.of tube 3Ewhich becomes conductive. However, triode 36 stays conductive onlyyashortiin- .terval of time, while the discharge (ii-condenser 39 through resistor lll holds the gridrof triodefl negative to cutoff. As soon as condenser 393$ discharged the potential on the grid of tube 142 'risesand the .tube '42 becomes conductive and a1- most immediately thereafter tube 36 becomes conductive againisince its grid'is 'still'held 'a'tga high positive value by the-signal marking shown .at 4'! .in. Fig. 3a. The .refiring of tube "4:2'cuts "tube 3.6 oif momentarilybyvirtuegof :the purrent -nal-frorn operating the triggering circuit. By proportioning condenser 39 and resistor 45 that tube 36 isin theconducting-state a much larger proportion of the time *than tube 42, the holdjfto the'gr'id 'of'tube'Sl "isof a greater average value. This hold back voltage is filtered by means of :condenser 43. 'These unequalconduction periods oftriodesi'lii and42'also supply a'higher average 'vaiueiofpotential;fi teredbycondenser 44, to the cathode of diode 2|. "Making'the value of capacitor 39 smaller'w'ith respect'to the value of '.resistor 40 'shortens the time required for the condenser to discharge, thereby making tube 42 conductive for -a smaller portion of the oscillatfing period :oftubes -36 and 42. The embodiment of 2 otherwise operates :substantially as describedabove'in Fig. l.

":Inithe embodiment of Fig. 4, the output of the ttube'42iis converted directly-into tone by means gofithe 'circuitshown. Condenser-G2 and inductan-ce iil comprise aparallelresonant circuit tuned 'to-the same frequency as that at which the flipflop tubes 36 and 42 oscillate. An isolating and can feed :resistor "allows the rectangular output "wavep'ftriode *42t0 be converted into sine wave form by theresonantcircuit (ii-'62. The cir- -*cuit"of-Fig.2 -'may then -be'broken at the points "oand'P'and tubeZl', elements 19', 20, 23', 24',

m5 25, "2-6"and 58"'and "capacitor-4'4 replaced by the resonant 'c ircuit-of Fig. '4. Figfi3e illustrates the wave form "of the ;output'of the circuit of "Fig. 4. Thecircuitieonstants of the system may be adjustedto give anydesired*tonefrequency. Then 10 the keyed tone output would be supplied over lines to 'a-rem'ote -point for "operating printing or *other recording equipment.

What isolainred ist 1. :Inasignalling'systemm pair 'o'f electron dis- .from tube 42 going through :common'cathodereidtharge devices waving-electrodes -"cro'ss :couple'd and connected to potential supply sources in such a manner that when one device is nonconductive the other device is conductive and vice versa, a control tube having its anode coupled to the control grid of said one device and having its control grid biased by such a potential that when the said control tube is conductive said one device is nonconductive and vice versa, a source of control potential which varies between two values in accordance with signals and also varies about said two values in an undesired manner, connections from said source to said control grid of the control tube for impressing said control potential thereon to change the conductivity of said control tube, when said control potential reaches one of said two values, a holding circuit connecting one electrode of one device to the control grid of said control tube and responsive to current fiow in said last named one of said two devices to apply additional potential to the control'grid of said control tube to retain the same in the state of conductivity to which it has been tripped for a time after said control potential applied to said grid recedes from said value at which the tubes conductivity changed, and an output circuit coupled to one of the devices.

2. A signalling system as recited in claim 1, wherein the coupling between said one device and said output circuit includes a potential limiter.

3. In signalling apparatus, a pair of electron discharge devices having electrodes including anodes, cathodes and control grids cross coupled and connected to potential supply sources in such a manner that when one device becomes nonconductive the other device becomes conductive and vice versa, a control tube having an anode, a cathode and a control grid, means coupling the anode of the control tube to the control grid of said one device, a circuit for biasing the control tube control grid relative to its cathode by a potential such that the tube is conductive, the arrangement being such that said one device is then non-conductive, connections for applying. a variable control potential to the control grid of the control tube which rises to a value such as to make the same non-conductive to thereby make said one device conductive, means responsive to current flow in said one device for applying additional potential to the control grid of said control tube to maintain the same non-conductive for limited variations of said control potential with respect to said value at which the tube became non-conductive, and an adjustable resistor connected to the cathode of the control tube for adjusting the potential at which the cathode of said control tube operates to adjust the level at which the control potential applied to the control tube grid becomes effective to make the same non-conductive.

4. In signalling apparatus, a pair of electron discharge devices having electrodes including anodes, cathodes and control grids cross coupled and connected to potential supply sources in such a manner that when one device becomes nonconductive the other device becomes conductive and vice versa, a control tube having an anode, a cathode and a control grid, means coupling the anode of the control tube to the; control grid of said one device, a biasing circuit for biasing the control tube control grid relative to its cathode by a potential such that the tube is conductive,

of the control tube which rises to a, value 'suchas' to make the same non-conductive to thereby make said one device conductive, means responsive to current flow in said one device for applying additional potential to the control grid of said control tube to maintain the same non-conductive irrespective of variations other than signal variations of said control potential below said value at which the tube became non-conductive, and adjustable resistive means in the grid circuit of said control tube for adjusting the effectiveness of. the additional potential applied to the control tube grid to thereby adjust the level to which the control potential applied to the control tube grid must fall to make the control tube conductive. 5. In signalling apparatus, a pair of electron discharge devices having electrodes including anodes, cathodes and control grids cross coupled and connected to potential supply sources in such a manner that when one device becomes non-conductive the other device becomes conductive and vice versa, a control tube having an anode, a cathode and a control grid, means coupling the anode of the control tube to the control grid of said one device, means for biasing the control tube control grid relative to its cathode by a potential such that the tube is conductive, the arrangement being such that said one device is then non-conductive, means for applying a variable control potential to the control grid of the control tube which rises to a value such as to make the same non-conductive to thereby make said one device conductive, means responsive to current flow in said one device for applying additional potential to the control grid of said control tube to maintain the same non-conductive for a time after said control potential applied to said control grid falls below said value at which the tube became non-conductive, adjustable resistive means for adjusting the potential at which the cathode of said control tube operates to adjust the level at which the control potential applied to the control tube grid becomes efiective to make the same non-conductive, and adjustableresistive means in the grid circuit of the control tube for adjusting the effectiveness of the additional potential applied to the control tube grid to thereby adjust the level to which the controlv potential applied to the control tube grid must fall to make the control tube conductive.

6. In signalling apparatus in combination, a pair of electron discharge devices having anodes, cathodes and control grids cross coupled by resistance and capacity and connected to potential supply sources in such a manner that when one device is non-conductive the other device is conductive and vice versa, the rate of reversal in the conditions of said devices depending in part at least on the amount of resistance and capacity in the circuits, an electron control device having a control electrode. and having an output electrode coupled to the control grid of one device in such a manner that the said control device and said one device are alternatively conductive, a source of potential which varies in accordance with signals between two values, means for applying said potential to the control electrode of said control tube to control the state of conductivity thereof, resistive means coupling the anode of said one device to the control electrode of said control device, and a parallel circuit tuned to the frequency of operation of said discharge devices connected with the anode of one of said discharge devices.

'1, In signalling apparatus, a pair of electron discharge devices each having an anode, a cathode, and a control grid with the electrodes interconnected and supplied by operating potentials in such a manner that when one device is nonconductive the other is conductive and vice versa, 7

a resistor in the supply circuit of said one device, a control tube having its anode coupled to the control grid of said one device and having its control grid normally biased to such a potential that the control tube is conductive, said one device and said control tube being alternatively conductive, a source of potential which varies from about zero potential to a negative value in accordance with signals and also varies about said negative value in accordance with noise or similar disturbances, means for applying said potential to the control grid of the control tube to bias the same to cutoff to trip the state of conductivity of the two devices, and a resistor coupling the anode of said one device to the control grid of said control tube to apply thereto the potential on the anode of said one device to supplement the bias on the control grid of said control tube to prevent subsequent variations in said control potential in accordance with noise or similar disturbances from retripping the state of conductivity of said two devices.

8. In signalling apparatus, a pair of electron discharge devices, each having an anode, a cathode, and a control grid, with the anodes and control grids cross-coupled and with the grids and anodes coupled to potential supply sources such that when one device is non-conductive the other device is conductive and vice versa, a resistor in the coupling between each anode and its supply source, a control tube having its anode coupled to the control grid of said one device and having its control grid in a biasing circuit such that when the control tube is conductive one of said devices is non-conductive and vice versa, a source of control potential which varies between two values in accordance with signals and also varies in an undesired manner, connections from said source to said control grid of the control tube for impressing said control potential thereon to change the state of conductivity of the control tube, when said control potential reaches one of said two values, a connection from the anode of said one device to the control grid of the control tube for applying additional potential to the control grid of said control tube to retain the same in the state of conductivity to which it has been tripped irrespective of variations, other than signal variations, of said control potential applied to said grid with respect to said value at which the tubes conductivity is changed, and an output circuit coupled to one of said devices.

9. In signalling apparatus, in combination, a locking circuit including electronic means with electrodes including an output electrode and an input electrode with the electrodes interconnected and supplied by operating potentials in such a manner that the locking circuit has two conditions of stability, a control tube having a control grid and having an anode coupled to said input electrode and having its control grid in a biasing circuit such that when the control tube is conductive the locking circuit is in one condition of stability and when the control tube is non-conductive the locking circuit is in the other condition of stability, a source of potential which varies in accordance with signals and also varies to a less extent in accordance with noise or similar disturbances, a connection between said source and said control grid of said control tube to control the state of stability of said locking circuit, and a coupling between said output electrode and an electrode of said control tube to apply thereto a potential which supplements the action of said control potential on the control grid of said control tube to prevent variations in said control potential in accordance with noise or similar disturbances from taking over control of the condition of stability of said locking circuit.

10. In signalling apparatus, in combination, a locking circuit including electronic means with electrodes including an output electrode and an input electrode with the electrodes interconnected and supplied by operating potentials in such a manner that the locking circuit has two conditions of stability, a control tube having a control grid and having an anode coupled to said input electrode and having its control grid normally biased to such a potential that the control tube is conductive, a source of potential which varies from about zero potential to a negative value in accordance with signals and also varies about said negative value in accordance with noise or similar disturbanes, means for applying said potential to the control grid of the control tube to bias the same to cut-off to trip the condition of stability of said locking circuit, and resistive means coupling said output electrode to the control grid of said control tube to apply thereto a potential which supplements the bias on the control grid of said control tube to prevent subsequent variations in said control potential in accordance with noise or similar disturbances from retripping the condition of stability of said locking circuit.

11. In signalling apparatus, a pair of electron control devices having electrodes cross coupled and connected to potential supply sources in such a manner that when one device is non-conductive the other device is conductive and vice versa, an electrode structure having its output electrode coupled to the control electrode of said one device and having its control electrode biased by such a potential that when the electrode structure is conductive said one device is non-conductive and vice versa, means for applying a control potential to the control electrode of the electrode structure of a value such as to make the same nonconductive, a connection between an electrode of said one device and the control electrode of the electrode structure whereby when current flows in said one device a holding potential is applied to the control electrode of said electrode structure to maintain the same non-conductive for limited variations of said control potential with respect to said value at which the electrode structure became non-conductive, and an output circuit coupled to one of said devices.

WARREN HERBERT BLISS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,061,734 Kell Nov. 24, 1936 2,185,199 Kahn Jan. 2, 1940 2,266,668 Tubbs Dec. 16, 1941 2,324,314 Michel July 13, 1943 2,383,126 Hollingsworth Aug. 21, 1945 2,426,454 Johnson Aug. 26, 1947 

